This invention relates to novel powders comprising coatings of an electrically conductive silicide, carbide, or boride on the surface thereof.
There has been a great deal of inventive effort devoted to producing small, electrically-conductive metal powders for use in electromagnetic energy shielding applications. One of the primary problems to be overcome was to develop a substitute for expensive silver loadings. To this end a number of substitutes were suggested, e.g. silver-coated copper and silver-coated glass. The former material has proved to be the choice in most applications because of its lower cost and high metal content. Nevertheless, because copper tends to migrate into and through the silver powder, eventually form an oxide of high electrical resistivity, work continues in an attempt to provide a suitable alternative to the favored approach.
In a different field of technology other investigators have been working to provide improved ferromagnetic powders for use in magnetic recording media. A great deal of work has been done in an attempt to provide particles of good corrosion resistance and of high magnetic moment. Iron, a particularly desirable candidate for such applications, is not favored because of its susceptibility to corrosion. Therefore most magnetic recording members have been made out of iron oxides. Such relatively exotic materials as "chromium dioxide" and high-cobalt alloys also have been developed for use in magnetic applications. However, where iron itself has been utilized, its potential effectiveness is grossly reduced because of the need to use an extraordinary amount of chemical stabilizers in the formation. Some patents generally descriptive of the work being done in this magnetic recording field include U.S. Pat. Nos. 3,649,541; 3,810,840; 3,586,630; 3,740,266; 3,149,995; 3,650,828; 3,630,771; 3,597,273 and many others.
Materials of improved properties are sought for use not only in the fields described above but also in making conductive formulations for use as flowable "solders", e.g. epoxy solders filled with silver, and the like. The achievement of providing a relatively inexpensive, chemically inert, powder of suitable electrical conductivity or suitable ferromagnetic character has eluded investigators. Gold and silver are still used when excellent chemical resistance or chemical stability are required.
In a hindsight search of prior work, a search made in view of the invention disclosed in this application, it was noted that carbide powders have been used as superconductive fillers (U.S. Pat. No. 3,380,935 to Ring), as fillers in cermets (U.S. Pat. No. 3,723,359), as a conductor in a ceramic material, as non-contacting, yet conductive, particles in a matrix to form a lossy dielectric material. None of these applications suggest the use of carbides or like materials as protective coatings which utilize the morphology of the coating to (1) protect the particulate mass or substrate and (2) to preserve the conductivity of the composition as a whole.
U.S. Pat. No. 3,671,275 to Gates wherein reflection of microwaves is said to be experienced at the expense of absorbing energy when conductive particles are in particle-to-particle contact. Gates relies on relatively small eddy current losses in large SiC particles to absorb energy. Such effects are relatively small when compared to the energy absorption achievable with the magnetic and conductive powders described hereinbelow.